Ehrlichia bacteria are obligate intracellular pathogens that infect circulating lymphocytes in mammalian hosts. The most natural mode of Ehrlichia transmission is via a variety of tick vectors. Ehrlichia canis (E. canis) and Ehrlichia chaffeensis (E. chaffeensis) are members of the same sub-genus group of Ehrlichia that infect canines and humans and cause canine monocytic ehrlichiosis (CME) and human monocytic ehrlichiosis (HME), respectively. Another species of Ehrlichia known as Ehrlichia ewingii (E. ewingii) has tropism for granulocytes and causes granulocytic ehrlichiosis. The canine disease is characterized by fever, epilepsy, incoordination, lethargy, bleeding episodes, lymphadenopathy, weight loss, and pancytopenia. In humans the disease is characterized by fever, headache, myalgia, and leukopenia.
Indirect immunofluorescence assays (IFA) and enzyme-linked immunosorbent assays (ELISA) have typically been used in the diagnosis of these diseases. These assays measure or otherwise detect the binding of anti-Ehrlichia antibodies from a subject's blood, plasma, or serum to infected cells, cell lysates, or partially purified whole Ehrlichia proteins. However, currently known assays for detecting anti-Ehrlichia antibodies or fragments thereof are severely limited in usefulness because of sensitivity and specificity issues directly related to the impure nature of the Ehrlichia antigen(s) used in these tests.
The diseases caused by bacteria belonging to different Ehrlichia species manifest differently and require separate management routine (Thomas, R. J., et al.; Expert Rev Anti Infect Ther. 2009 August; 7(6): 709-722). It is, therefore, important to identify the Ehrlichia species that causes a particular infection. The currently known immunoassays use mixtures of many whole Ehrlichia antigens or antigens that are not species specific. PCR methods, which may be useful to identify Ehrlichia species, are useable only if the tick is recovered and/or the tissue from host is tested soon after infection. Furthermore, cultivation of bacteria from the infection site, another method which may be useful to identify Ehrlichia species, is not only technically complex but also requires freshly infected tissue. In addition, a cultivation method for the species E. ewingii has not yet been developed.
Accordingly, there remains a need in the art for additional assays for detecting Ehrlichia antigens and serodiagnosis of monocytic ehrlichiosis and granulocytic ehrlichiosis. In particular, there remains a need for an assay for identifying Ehrlichia species, especially an assay that can be used in a variety of circumstances and for various samples, including samples that do not require isolation from freshly infected tissues. The present invention provides methods, compositions, and kits to facilitate the diagnosis, the species identification, and the treatment of the various types of Ehrlichia infections.